1. Field of the Invention
This invention relates to holography. More particularly, it relates to wavelength scanning digital interference holography.
2. Description of the Prior Art
Dennis Gabor is credited with the invention of holography in 1948 while attempting to improve the resolution of electron microscopy. At the time, however, his invention could not be made practical because there were no light sources available with the required coherence. The invention of laser and the introduction of off-axis holography provided the critical elements to make holography practical and a powerful tool for large areas of applications from metrology, data storage, optical processing, device fabrication, and even fine arts. The conventional process of holography using photographic plates, however, is time-consuming and cumbersome. Real time process is not feasible unless photorefractives and other nonlinear optical materials are used. Recently, the field of holography has been undergoing another paradigm shift by electronic image capture using CCD array cameras and digital processing of holographic images.
By recording the phase as well as intensity of light wave, holography allows reconstruction of the images of three-dimensional objects, and gives rise to a host of metrological and optical processing techniques. With the advance of computer and electronic imaging technology, it is now very practical and often advantageous to replace portions of the conventional holographic procedures with electronic processes. In digital holography, the holographic interference pattern is digitally sampled by a CCD camera and the image is numerically reconstructed by applying the results from the diffraction theory. This offers a number of significant advantages such as the ability to acquire the images rapidly, the availability of both amplitude and phase information of the optical field and the versatility of the processing techniques that can be applied to the complex field data.
Moreover, advances in digital imaging devices such as CCD and CMOS cameras and in computational and data storage capacities have been central to the widening applications of digital holography. Microscopic imaging by digital holography has been applied to imaging of microstructures and biological systems. In digital holography, the phase of the optical field, as well as the amplitude, results directly from the numerical diffraction of the optically recorded holographic interference pattern and leads to images of axial resolution at a mere fraction of wavelength. This can be used for numerical corrections of various aberrations of optical systems such as field curvature and anamorphism. In microscopy applications, the reconstructed image can be numerically focused to any plane in the object.
Images may also be advantageously reconstructed along an arbitrarily tilted plane. In most 3D microscopy systems, including optical coherence tomography (OCT) and wavelength scanning digital interference holography (WSDIH), the 3D volume is reconstructed as a set of scanning planes with the scanning direction along the optical axis of the system. The plane on which the reference mirror is located is called the scanning plane and its normal direction is defined as the scanning direction. If a tomographic image on a plane not parallel to the original reference mirror is required, it can be reconstructed by combining or interpolating points from different tomographic layers. The quality will be degraded, however, especially when the lateral resolution does not match well with the axial resolution. To get better results, the whole process needs to be physically repeated with the reference mirror tilted or the object rotated to a desired orientation.
The prior art includes a zero padding method to control the resolution for the FDF, where the new resolution is decreased by adding more zeros to increase the total pixel number. However, this method cannot be used to adjust the pixel resolution for a distance smaller than zmin. The prior art further includes a double-Fresnel-transform method (DFTM) to adjust the reconstruction pixel by introducing a transitional plane (TP) and implementing the FDF twice. The final resolution is proportional to the ratio |z2|/|z1|, where |z2| is the distance from the TP to the destination plane (DP) and |z1| is the distance from the hologram to the TP, with |z1|, |z2|≧zmin. If the object-to-hologram distance is small, the above ratio can be adjusted only in a limited range. Specifically, the DFTM can not be used for resolution control if the DP is close to the hologram.
An improved system that overcomes these shortcomings is needed.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the identified needs could be met.